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Photocatalytic splitting of hydrogen sulfide(H2S)for hydrogen evolution is a promising method to solve the energy and environmental issues.In this work,S,N-codoped carbon dots(S,N-CDs)/graphitic carbon nitride(g-C3N4)nanosheet is synthesized by hydrothermal method as an efficient photocatalyst for the decomposition of H2S.In addition to the characterization of the morphology and structure,chemical state,optical and electrochemical performances of S,N-CDs/g-C3N4,hydrogen evolution tests show that the activity of g-C3N4 is improved by introducing S,N-CDs,and the enhancement depends strongly on the wavelength of incident light.The photocatalytic hydrogen production rate of S,N-CDs/g-C3N4 com-posite reaches 832 pmol g-1h-1,which is 38 times to that of g-C3N4 under irradiation at 460 nm.Density functional theory calculations and electron paramagnetic resonance as well as photolumines-cence technologies have altogether authenticated that the unique wavelength-dependent photosensitiza-tion of S,N-CDs on g-C3N4;meanwhile,a good match between the energy level of S,N-CDs and g-C3N4 is pivotal for the effective photocatalytic activity.Our work has unveiled the detailed mechanism of the photocatalytic activity enhancement in S,N-CDs/g-C3N4 composite and showed its potential in photocat-alytic splitting of H2S for hydrogen evolution.